Ensure sparse files on windows during download

The file.Truncate call on windows will write the whole file
unless you set the sparse flag, leading to heavy I/O at the
beginning of download.  This should improve our
I/O behavior on windows and put less stress on the users disk.

server/download.go

@@ -216,6 +216,9 @@ func (b *blobDownload) run(ctx context.Context, requestURL *url.URL, opts *regis
 		return err
 	}
 	defer file.Close()
+	if err := setSparse(file); err != nil {
+		return err
+	}
 
 	_ = file.Truncate(b.Total)